Recently, a new class of engineering thermoplastics has been introduced based upon thermotropic liquid crystal polymers which combines the desirable attributes of moldability with multidirectional mechanical strength superior to conventional thermoplastic materials. Generally, these liquid crystal polymers are polyesters made up of planar, linear disubstituted aromatic compounds, for example, p-hydroxybenzoic acid, p-hydroquinone, 4,4'-dihydroxybiphenyl and 2-hydroxy, 6-naphthenoic acid.
Further commercial opportunities for developing new liquid crystal polymers are largely dependent upon the availability and price of feed stock materials. Certain liquid crystal polymers would be commercially attractive if 2,6-dihydroxynaphthalene or 2,6-dicarboxynaphthalene were readily available. While these feedstocks are not currently readily available, a potentially viable precursor is 2,6-diisopropylnaphthalene, which can be easily converted to the dihydroxy- or dicarboxy- forms based upon existing technology. The present invention relates to improvements in the manufacture of 2,6-diisopropylnaphthalene from naphthalene.
Before proceeding with an explanation of these improvements, it is first necessary to establish terminology and a thorough understanding of the isopropylation reaction scheme. The chemical structure which follows shows the positional reference numbers for various substituted naphthalene isomers. Non-hydrogen-bearing carbons are unnumbered because no substitution takes place in these positions. ##STR1##
There are two possible isomers which are formed in the monoisopropylation of naphthalene. Substitution occurs only in the 1 and 2 positions and is respectively denoted .alpha. and .beta.. Any monoisopropyl substitution which takes place in positions 3 through 8 is identical to the .alpha. and .beta. positions due to their interrelationship in symmetry.
Multiple naphthalene isopropylation is usually denoted by the position number. Some literature references follow the numbering convention just described, while other references discuss the isomers in terms of the a and b terminology. Thus, the 2,6 isomer is the double .beta. isomer.
Table 1 describes the statistical distribution of the various diisopropylates using these designations. There is listed no 1,2-, 2,3-, and 1,8-diisopropylnaphthalene for orthodiisopropylation does not occur due to steric effects. In Table 1, it is shown that there are seven disubstituted isomers, of which two (2,6- and 2,7-) are the double .beta. product.
TABLE 1 ______________________________________ NORMALIZED TYPE ISOMER EQUIVALENTS FREQUENCY % ______________________________________ .alpha.,.beta. 1,6 1,6 = 4,7 = 2,5 = 3,8 20 1,7 1,7 = 4,6 = 3,5 = 2,8 20 1,3 1,3 = 2,4 = 5,7 = 6,8 20 .alpha.,.alpha. 1,4 1,4 = 5,8 10 1,5 1,5 = 4,8 10 .beta.,.beta. 2,6 2,6 = 3,7 10 2,7 2,7 = 3,6 10 ______________________________________
In the manufacture of diisopropylnaphthalene, it is clear that some monoisopropyl- and triisopropyl products, and a mix of diisopropyl isomers, will also be obtained. In any crude diisopropylnaphthalene product which is not particularly enriched in the desired 2,6-diisopropylnaphthalene isomer, separation of this isomer by thermal distillation is very inefficient and difficult because the boiling points of 2,6-diisopropylnaphthalene and 2,7-diisopropylnaphthalene are very close. Similarly, 2,6-diisopropylnaphthalene isomer separation by fractional crystallization using melting points is inefficient and suffers from yield problems because of the loss of the desired product in the mother liquor, and because of resulting large recycle streams
A prior art process for producing 2,6-diisopropylnaphthalene from naphthalene is shown in FIG. 2, labeled "Prior Art." The three main components of this process are alkylation reactor 2, 2,6-diisopropylnaphthalene separation means 4, and transalkylation reactor 6. Propylene feed stream 8 and fresh naphthalene feed stream 10 are contacted in an alkylation reactor 2 containing a solid acid catalyst. Diisopropylnaphthalene product 12 is combined with a transalkylation recycle stream 14 to form stream 16, which is fed to 2,6-diisopropylnaphthalene separation means 4.
2,6-diisopropylnaphthalene separation means 4 can be a combination of unit operations which increase the purity of the desired 2,6-diisopropylnaphthalene product. In one such combination, thermal distillation can be used to separate naphthalene and monoisopropylnaphthalene from the higher boiling polyisopropylnaphthalenes. Thermal distillation can also be used to separate tri- and tetraisopropylnaphthalenes from the diisopropylnaphthalenes. Finally, crystallization or selective adsorption can be used to recover substantially pure 2,6-diisopropylnaphthalene from the other diisopropylnaphthalene isomers. The 2,6-diisopropylene product is represented as stream 24 in FIG. 2. Naphthalene and monoisopropylnaphthalene recycle stream 18, diisopropylnaphthalene recycle stream 20, tri- and tetraisopropylnaphthalenes stream 22 and napthalene feed stream 26 are fed to transalkylation reactor 6 containing a solid acid catalyst. The transalkylation reaction product, transalkylation recycle stream 14, is typically rich in diisopropylnaphthalene and is typically at equilibrium, being characterized by a 2,6/2,7-diisopropylnaphthalene molar ratio of about 1 and a diisopropylnaphthalene fraction containing no greater than 39 mole percent 2,6 diisopropylnaphthalene. Since a non-shape selective catalyst is used in the alkylation reactor, the composition of diisopropylnaphthalene product 12 will be similar to stream 14 and when combined, the feed to 2,6 diisopropylnaphthalene separating means 4 will also have a 2,6/2,7 ratio of about 1 and contain about 39 mole percent 2,6 diisopropylnaphthalene in the diisopropylene fraction.
Even if a shape selective catalyst were used for the alkylation step to produce an alkylate stream 12 with a 2,6/2,7 diisopropylnaphthalene molar ratio greater than 1.0 and a diisopropylnaphthalene fraction containing greater than 39 mole percent 2,6-diisopropylnaphthalene, the beneficial effect of the shape selective alkylation would be greatly diminished by this process configuration. The recycle stream 14 will typically be at equilibrium, with a 2,6/2,7 diisopropylnaphthalene molar ratio about 1.0 and a diisopropylnaphthalene fraction containing about 39 mole percent 2,6 diisopropylnaphthalene. Since the recycle stream is typically much larger than stream 12, when the two streams are combined and fed to separating means 4, this stream 16 will have only marginally enhanced 2,6/2,7 diisopropylnaphthalene molar ratio and 2,6 diisopropylnaphthalene content in the total diisopropylnaphthalene.
Therefore, it is an object of this invention to provide a process for the selective manufacture of 2,6-diisopropylnaphthalene which enables more efficient separation of the desired 2,6-diisopropylnaphthalene from other alkylation reaction products than prior art methods.
It is another object of the present invention to provide an alkylation reaction product which is enriched in both total diisopropylnaphthalenes and in the mole ratio of 2,6-diisopropylnaphthalene to 2,7-diisopropylnaphthalene.
It is still another object of the present invention to provide an integrated process scheme in which essentially all of the fresh naphthalene feed is converted to the desired 2,6-diisopropylnaphthalene product resulting in a high process yield and low raw material cost.
These and further objects of the present invention will become apparent to those of ordinary skill in the art with reference to the figures and detailed description below.